1. Field of the Invention
This invention relates to lubricating oil compositions, their method of preparation and use. Specifically, this invention relates to lubricating compositions that contain a metal compound and a hindered amine. The use of a metal compound and the hindered amine act synergistically to surprisingly provide protection of the lubricant from oxidation. The addition of an aromatic amine, particularly a diarylamine, to this combination provides even better protection.
2. Description of the Related Art
Oxidation is a major cause of the breakdown of lubricants. This results in a shortened lifespan of the lubricant, requiring more frequent changes, especially in demanding environments such as internal combustion engines.
Antioxidants have therefore played an important role as additives in lubricants in order to extend their useful life. Aromatic amines, especially secondary diarylamines, e.g., alkylated diphenylamines, phenothiazines, and alkylate N-naphthyl-N-phenylamines, have been important additives to lubricating compositions. Also important have been phenolic compounds in retarding oxidation.
The combination of an antioxidant with a metal compound has been important in extending the lifetime of the antioxidant. For example, U.S. Pat. No. 5,994,277 to Richie et al. teaches that a crankcase lubricant composition which contains copper, molybdenum and aromatic amines can act as an effective antioxidant combination. U.S. Pat. No. 6,306,802 to Shaub et al. discloses sulfurized molybdenum complexes with oil-soluble aromatic amines. Gatto, et al., in U.S. Pat. No. RE38,929E has disclosed that the combination of certain sulfur and phosphorus-free molybdenum compounds and secondary diarylamines improved the useful life of a lubricating oil. The most effective amounts in inhibiting oxidation were between 100 and 450 parts per million (ppm) of molybdenum, and between 750 and 5,000 ppm of an oil-soluble secondary diphenylamine.
Other combinations of antioxidants have also been used. U.S. Pat. Nos. 5,073,278 and 5,273,669 to Schumacher et al. disclose the synergistic combination of aromatic amines and hindered amines in a lubricating oil. U.S. Pat. No. 5,268,113 to Evans et al. discloses the combination of a hindered amine with phenolic compounds.